Despite the rather rocky history of fantasy fiction pulps, with the failures of largely-fantasy [[ThrillBook The Thrill Book and Astoundingís former Clayton stablemate Strange Tales, along with the continuing marginal commercial status of Weird Tales, Campbell apparently made the argument that work such as Eric Frank Russellís Sinister Barrier, the lead novel in the first issue (dated March), would be better presented in a fantasy setting rather than as science fiction in Astounding. Considering the thread of fantasy and near fantasy Campbell would continue to publish in /Astounding after Unknownís folding in 1943, it was clear that his heart was at least as much with the kind of ìrational fantasyî he published in Unknown as with the ìhardî science fiction he is best-remembered for.

Sinister Barrier wouldn't have been too out of place in Astounding, but the first issue also boasted a fine grim Manly Wade Wellmanhorror story, ìWhere Angels Fear,î and H. L. Goldís ìTrouble with Water,î a fresh variation on the kind of humorous fantasy Thorne Smith was publishing to great success in book form. This last vein was probably the mode Unknown has been best remembered for, stories which followed H.G. Wellsís dictum that a fantastic story should have only one miraculous situation in it and retain as much realism as possible around that miracle; Campbell published a number of similar works, particularly by L. Sprague de Camp alone and in collaboration, which featured a fairly rigoruous working-out of the limits of the fantasy devices employed.

Leiberís groundbreaking urban horror story ìSmoke Ghostî and first novel ìConjure Wifeî were among the most important examples of the macabre Unknown would publish; like Leiber, another new Campbell star, Theodore Sturgeon was also publishing in Astounding but seemed at least as much at home in Unknown, offering among many others such influential horror stories as ìItî and ìShottle Bop,î and slightly more surreal exercises such as ìThe Ultimate Egoistî and ìYesterday was Monday.î In fact, most of Campbellís favorites in Astounding, including Robert Heinlein, A. E. van Vogt, Cleve Cartmill, Henry Kuttner and others, would also publish in Unknown.

The magazine gave up on cover paintings with the 17th issue, July 1940, opting instead for a rather reserved cover template, with several story titles and their authors listed with a small spot illustration next to each title; the magazine also went at this point from standard pulp size to bedsheet size. With the 27th issue, October 1941, the title change to Unknown Worlds was effected, perhaps in hopes of drawing more science fiction-oriented readers; apparently, sales didn't warrant its continuation during World War IIís paper shortages, and it folded with the October 1943 issue, its 39th.

Street and Smith tested the market again in 1948, with an all-reprint special annual titled From Unknown Worlds, but did not pursue the project further.

Many stories published in the magazine have been very widely reprinted since, and several further anthologies have been drawn from its pages, most influentially D.R. Bensenís The Unknown and its sequel, The Unknown Five, which printed for the first time an Isaac Asimov story that had been in UWís inventory when it folded.